replace GPU in 2010 Mac pro?
 

I have a late 2010 2.8 Ghz Quad-core 24GB RAM Mac Pro with the ATI Radeon HD 5770. I am using Premiere CC (and AE CC a bit as well) and feeling like there is a lot of GPU power that I could be harnessing if my card was supported, and perhaps more if I went the CUDA route. Enabling GPU acceleration seems to slow things down, if anything, with my current card.

I'm interested in spending up to $300 on a graphics upgrade.

The work I do is mostly 1080P AVCHD editing with lots of color effects and overlays, especially using Colorista II. I get a lot of halting playback that is mostly unwatchable unless I render (obviously) or play the same section of video a few times.

Any suggestions on a card to upgrade to? Or some thoughts on whether upgrading is even worth it? (I have doubts about anyone being willing to buy my used 5770.)